CVE-2021-33586: InspIRCd 3.8.0 through 3.9.x before 3.10.0 allows any user (able to connect to the server) to access recently

Routine

MEDIUM 4.3UPGRADE

CVE-2021-33586 is a medium-severity flaw (CVSS 4.3) affecting inspircd. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

InspIRCd 3.8.0 through 3.9.x before 3.10.0 allows any user (able to connect to the server) to access recently deallocated memory, aka the "malformed PONG" issue.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.9% (top 41%)
CISA exploited list	No
Weakness	CWE-732
Published	2021-05-27
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: inspircd

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-05-27 05:15 UTCPublished CVE record published
2026-06-17 03:54 UTCUpdated Record last modified

inspircd inspircd UPGRADE

Affected: >= 3.8.0 and < 3.10.0

Fixed in: 3.10.0

Safe to downgrade to: a release before 3.8.0

1. Check installed version

apt-cache policy inspircd || rpm -q inspircd || inspircd --version

2. UPGRADE to 3.10.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade inspircd

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade inspircd

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.8.0 (Debian / Ubuntu)

apt-cache madison inspircd sudo apt install inspircd=<VERSION_BEFORE_3.8.0> sudo apt-mark hold inspircd

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list inspircd sudo dnf downgrade inspircd-<VERSION_BEFORE_3.8.0> sudo dnf versionlock add inspircd

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-33586?

inspircd: upgrade to 3.10.0. Exact commands for each OS are on this page.

Is CVE-2021-33586 being exploited?

CVE-2021-33586 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-33586 affect?

CVE-2021-33586 affects inspircd. Severity is medium (CVSS 4.3). InspIRCd 3.8.0 through 3.9.x before 3.10.0 allows any user (able to connect to the server) to access recently deallocated memory, aka the "malformed PONG" issue.

How urgent is CVE-2021-33586?

Priority 30/100. No elevated risk signals yet.

Related CVEs

CVE-2020-25269 MEDIUM 6.5 UPGRADE MITIGATE An issue was discovered in InspIRCd 2 before 2.0.29 and 3 before 3.6.0
CVE-2019-20918 MEDIUM 6.5 UPGRADE An issue was discovered in InspIRCd 3 before 3.1.0
CVE-2019-20917 MEDIUM 6.5 UPGRADE MITIGATE An issue was discovered in InspIRCd 2 before 2.0.28 and 3 before 3.3.0
CVE-2016-7142 MEDIUM 5.9 UPGRADE MITIGATE The m_sasl module in InspIRCd before 2.0.23, when used with a service that supports SASL_EXTERNAL authenticati
CVE-2015-8702 HIGH 8.6 MITIGATE UPGRADE The DNS::GetResult function in dns.cpp in InspIRCd before 2.0.19 allows remote DNS servers to cause a denial o

References

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